1. Field of the Invention
The present invention relates to improvements in a secondary air supply system which is used in a supercharged engine, and more particularly to such a secondary air supply system which improves engine performance in a high engine load and high engine speed condition.
2. Description of the Prior Art
It is well known to use a three-way catalytic converter (a device which is capable of simultaneously reducing the concentrations of CO, HC and NO.sub.x in the exhaust gases) in the exhaust system of automotive engines. Such a conventional system, however, cannot efficiently convert CO and NO.sub.x when an enriched air/fuel mixture is supplied to the engine under an idling condition. To efficiently convert the exhaust gases under the idling condition, it has been proposed that the exhaust system be provided with a secondary air supply arrangement wherein secondary air is supplied into the exhaust gases under an idling condition. Such an arrangement is disclosed, for example, in Japanese Patent Provisional Publication Nos. 53-76217 and 58-172415. The secondary air supply arrangement includes a diaphragm valve which opens a secondary air supply passage so that secondary air is supplied into the exhaust gases under an idling condition. The arrangement further includes a reed valve which operates to guide the secondary air into the exhaust gases under the pulsation of the exhaust gas.
However, when this conventional arrangement is used in a supercharged engine, a problem arises because the exhaust gas flows into the intake passage through the secondary air supply passage under a high engine load and high engine speed condition. This occurs sealing performance of the reed valve being not very good, the arrangement cannot prevent the exhaust gas from leaking at the reed valve. The diaphragm-operated valve is opened by communication between the pressure chamber of the diaphragm-operated valve and negative pressure in the intake passage, thereby guiding secondary air into the exhaust gas. The diaphragm-operated valve is closed by communication between the pressure chamber and the atmosphere under other than the high load and speed condition. Therefore, under a high pressure condition of the exhaust gas such as a high engine load and high engine speed condition, the exhaust gas leaking from the reed valve pushes and opens the diaphragm-operated valve. Furthermore, the exhaust gas flows into the engine through the air cleaner, thereby degrading the engine performance under a high engine load and high engine speed condition.